We are given the following unconstrained optimization problem:
\mathbf{x}^* = \arg \min_{\mathbf{x}} f(\mathbf{x})where f: \mathbb{R}^n \to \mathbb{R} is a differentiable objective function, and \mathbf{x} \in \mathbb{R}^n is the vector of decision variables.
The first-order necessary conditions for optimality are given by:
\nabla f(\mathbf{x}^*) = 0where \nabla f(\mathbf{x}) denotes the gradient of f(\mathbf{x}) .